inclusion criteria: * healthy male or female individuals aged 18 years to 60 years * participant is willing and able to give written informed consent for participation in the trial * individuals who can comply with trial procedures and are available for the duration of follow-up. brazil: ● previous vaccination with a complete primary series of sinovac (priming group 1), azd1222 (priming group 2), or bnt162b2 (priming group 3-b) at least 6 months prior to screening pakistan: ● previous vaccination with a complete primary series of sinovac (priming group 1) or azd1222 (priming group 2) at least 6 months prior to screening, or pcr-confirmed natural infection (priming group 3-p) between february 2021 - 6 months prior to screening

inclusion criteria: * healthy male or female individuals aged 18 years to 60 years * participant is willing and able to give written informed consent for participation in the trial * individuals who can comply with trial procedures and are available for the duration of follow-up. brazil: ● previous vaccination with a complete primary series of sinovac (priming group 1), azd1222 (priming group 2), or bnt162b2 (priming group 3-b) at least 6 months prior to screening pakistan: ● previous vaccination with a complete primary series of sinovac (priming group 1) or azd1222 (priming group 2) at least 6 months prior to screening, or pcr-confirmed natural infection (priming group 3-p) between february 2021 - 6 months prior to screening

inclusion criteria: healthy male or female individuals aged 18 years to 60 years participant is willing and able to give written informed consent for participation in the trial individuals who can comply with trial procedures and are available for the duration of follow-up. brazil: ● previous vaccination with a complete primary series of sinovac (priming group 1), azd1222 (priming group 2), or bnt162b2 (priming group 3-b) at least 6 months prior to screening pakistan: ● previous vaccination with a complete primary series of sinovac (priming group 1) or azd1222 (priming group 2) at least 6 months prior to screening, or pcr-confirmed natural infection (priming group 3-p) between february 2021 - 6 months prior to screening

inclusion criteria: healthy male or female individuals aged 18 years to 60 years participant is willing and able to give written informed consent for participation in the trial individuals who can comply with trial procedures and are available for the duration of follow-up. brazil: ● previous vaccination with a complete primary series of sinovac (priming group 1), azd1222 (priming group 2), or bnt162b2 (priming group 3-b) at least 6 months prior to screening pakistan: ● previous vaccination with a complete primary series of sinovac (priming group 1) or azd1222 (priming group 2) at least 6 months prior to screening, or pcr-confirmed natural infection (priming group 3-p) between february 2021 - 6 months prior to screening